Unfortunately, you just cannot go to a store and buy a single instrument to measure the dew point like you can when buying a thermometer. The reason for this is the dew point is a mathematical equation. Check out this link that offers several methods of measuring the dew point, none of which I like.
https://wxguys.ssec.wisc.edu/2011/08/01/how-is-the-dew-point-measured/
You can purchase instruments that will do the math for you and provide the dew point along with other weather data. Amazon.com Walmart, Home Depot, Lowe’s, and other stores sell devices that will get the job done. Make sure before you purchase it says on the package it will display dew point.
Read the fine print or call the company because some of them only display the dew point by pressing a separate button, unlike the temperature which is displayed continuously. The bad news is these instruments vary widely in terms of accuracy and range. I have found the real low-priced models say under $50.00 have limited range and accuracy. Do your research, read the reviews, before deciding on which one to purchase.
Friend’s and family members ask me from time to time what do I use to get weather data in my backyard? I own and highly recommend the Davis Instrument’s Vantage Vue. I have been using Davis products since the mid-1980s. In fact, I still have and use the Davis Instruments Weather Monitor II to get the dew point in my crawl space. It is a wired device, and it goes way back not sure how long, but it still works after many years. They no longer sell that as everything has gone wireless. Some might feel the Vantage Vue that I currently use is too pricey. It is priced between $267.00 to a high of $400.00 depending on where you buy it from. Considering how much data it provides and its rock-solid reliability I think its priced exactly right. (subject to opinion.) Adding software at additional cost will allow you to record an abundance of weather data including the inside as well as the outside dew point!! You can set the increments as to how often it records data. I have mine set to hourly. Below is the link that shows live readings from my backyard.
https://www.weatherlink.com/embeddablePage/show/4d077963475f4f649509097e342f43d2/fullscreen
Here is the link for Davis Instruments: https://www.davisinstruments.com/
These are the websites I like for weather data and forecast. The weatherchannel.com (#1 in my opinion) (offers a current dew point map) Weatherbug.com (the only web site I have found that provides future dew points. Click on hourly) Accuweather.com
For weather radar in addition to the above websites I use the apps from Radar X and Rainaware on my android smartphone.
